Contents lists available at ScienceDirect Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs TiO 2 hollow microspheres impregnated with biogenic gold nanoparticles for the efficient visible light-induced photodegradation of phenol I. Hazra Chowdhury, M. Roy, S. Kundu, M.K. Naskar * Sol-Gel Division, Central Glass & Ceramic Research Institute (CSIR), Kolkata, 700 032, India ARTICLE INFO Keywords: Photocatalysis Photoluminescence Microstructure Spectroscopic property TiO 2 ABSTRACT In this study, titania hollow microspheres were prepared via sol–gel process followed by autoclaving at 180 °C for 20 h using titanium (IV) oxysulfate and ammonium fluoride. Different amounts of Au in a water-dispersible sol obtained from carambola fruit extract at room temperature were incorporated into the synthesized TiO 2 to prepare Au/TiO 2 nanocomposite. The physicochemical properties of the synthesized products were studied by X- ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, N 2 adsorption–desorption analysis, ultraviolet-diffuse reflectance spectroscopy, photoluminescence spectro- scopy, field emission scanning electron microscopy, and transmission electron microscopy. The prepared Au/ TiO 2 nanocomposite was applied to study the photocatalytic degradation of phenol (a toxic organic pollutant) under irradiation by visible light. The Au/TiO 2 nanocomposite containing 5 wt% Au had the smallest band gap energy of 2.24 eV and the maximum photocatalytic efficiency (95%), where it decomposed 97.5% of the organic pollutant after irradiation by visible light for 1 h. 1. Introduction In materials science and technology, titania is of great interest to researchers because of its excellent photochemical properties and in- ertness. Titania has versatile applications as a photocatalyst as well as in solar cells, pigments, capacitors, and other uses [1]. The properties of materials govern their applications. The photocatalytic properties of titania are well-known and its photodegradation efficiency can be ad- justed by tuning its crystalline phase, porosity, and microstructures [2]. Nanostructured TiO 2 has been produced in various shapes such as rods, cubes, tubes, wires, and spheres [3–7]. However, the hollow spherical microstructure has received much attention because the empty space inside the hollow structure facilitates catalytic reactions due to the easy access and the liberation of reactant and product molecules to and from the reactive sites [8,9]. In general, titania is prepared using solution- based approaches, such as co-precipitation, sol–gel, hydrothermal/sol- vothermal, microemulsion, and microwave methods [10–15]. In the present study, we employed a synergistic method (sol–gel and hydro- thermal) to fabricate Au/TiO 2 nanocomposites. The pure TiO 2 has limited photocatalytic efficiency because of its broad band gap energy and the enhanced recombination of electron and hole pairs [16]. Therefore, instead of using pure TiO 2 , metal-decorated TiO 2 (Au/TiO 2 composites) is preferred because of its much greater photocatalytic efficiency, which can extend to light absorption in the visible range. However, most of the previously reported photocatalysts respond only to ultraviolet (UV) light irradiation (< 420 nm). The utilization of visible light is more important than UV light because 43% of the solar spectrum is in the visible light range. Therefore, the fabrication of visible light-driven photocatalysts is one of the most important and significant issue in this field. The incorporation of metals into TiO 2 dramatically changes its inherent properties and increases its applica- tion range. In particular, the presence of nano-plasmonic Au on the surface of TiO 2 has dual effects. The nano-Au exhibits strong localized surface plasmon resonance (LSPR) and hence it can be effectively exploited in the visible spectrum range. In addition, nano-Au acts as an electron trap to promote electron–hole separation [17]. Au-doped ti- tania has been synthesized using different methods for various appli- cations. Yogi et al. prepared Au-doped TiO 2 nanoparticles via the sol–gel method and demonstrated their photocatalytic properties [18]. Grirrane et al. synthesized Au/TiO 2 via the deposition–precipitation method for the aerobic oxidation of amines [19]. Au-doped TiO 2 na- noparticles were synthesized by Rahulan et al. via the sol–gel method to study their optical limits [20]. In the present study, we prepared Au nanoparticles with a green synthetic approach using green carambola fruit extract at room temperature, followed by incorporating them in the TiO 2 matrix. Green carambola is useful for hindering the oxidation https://doi.org/10.1016/j.jpcs.2019.01.036 Received 16 July 2018; Received in revised form 30 January 2019; Accepted 31 January 2019 * Corresponding author. E-mail address: milan@cgcri.res.in (M.K. Naskar). Journal of Physics and Chemistry of Solids 129 (2019) 329–339 Available online 01 February 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved. T